One method uses computer graphics to augment camera images, superimposing virtual objects of the target category onto the image [1]}, [2]}. This approach is simpler than synthesizing entire scenes and improves generalization. A second method is to build scenes and objects using game engine technology [3]}, [4]}. This approach is helpful for building scenes; its limitation concerns the image quality of rendering. Third, some authors use ray-tracing to produce high quality images [5]}, [6]}. Ray-tracing has been combined with driving simulators to automate the creation of large numbers of realistic scenes [7]}. A limitation of ray-tracing is that the rendering time is long compared to game engines, real-time ray-tracing is a current undergoing research topic [8]}, which can potentially be helpful.
